1. Field of the Invention
The present invention relates to an eye pattern display method and an eye pattern display apparatus for data transmission using a modem or the like. The invention also relates to a communications apparatus for use with the same.
2. Description of the Related Art
In data transmission using a modem, for example, data for transmission is encoded, before being transmitted, into signal points arranged in a two-dimensional or a complex plane (two-dimensional values or complex numbers). At the receiving end, the locations of the received signal points are determined, and based on the signal points thus determined, data demodulation is performed.
The number of signal points and their locations in the two-dimensional or the complex plane are specified for each of various transmission schemes. Under ideal conditions, the received signal points would always appear in their specified locations, but in the presence of channel characteristic degradation, the received signal points tend to be displaced from the specified locations because of the superposition of noise, the occurrence of phase shifting, etc. This tendency becomes more pronounced as channel characteristic degradation increases.
Since communication becomes impossible, for example, when the channel characteristics greatly degrade, it has been practiced to examine the reception condition of the modem and take necessary countermeasures.
To examine the reception condition of a modem, the modem is connected to an oscilloscope on which an eye pattern is displayed to view a displacement of signal points, and based on the amount of displacement, the degree of eye pattern degradation (channel characteristic degradation) is determined.
In recent years, there has been an increasing need for higher-rate transmission using modems. This can be attained by increasing the number of signal points to be transmitted. However, the number of signal points increases exponentially with increasing transmission rate; for example, 128 signal points are required for a transmission rate of 14.4 Kbps specified in the ITU-T recommendation V.33, and a maximum of 896 signal points for a transmission rate of 28.8 Kbps specified in V.34.
With low transmission rates, since the number of signal points required is small, the spacing of the signals displayed on an oscilloscope is relatively wide, so that the amount of displacement of the received signal points can be easily discerned by displaying the eye pattern on the oscilloscope.
However, as the number of signal points increases, the signal point spacing falls compared to low-rate transmission. As a result, on the eye pattern displayed on the oscilloscope, the signal points are displayed closer to each other. This tendency becomes more pronounced as the transmission rate is increased.
The closer spacing of the displayed signal points makes it difficult to discern whether each displayed signal point is located in its intended position or is displaced from that position, and if displaced, how much it is displaced. The result of this is the inability to accurately examine the degradation of the eye pattern.